1. Field of the Invention
The present invention relates to a capacitor structure, and in particular, is applied to a semiconductor memory using a capacitor as a data storage element.
2. Description of the Related Art
A capacitor used for a semiconductor integrated circuit has a structure, for example, as shown in FIG. 1. In recent years, in order to make the capacitance of a capacitor large, a case in which a high-dielectric is used as a dielectric (inter-electrode dielectric) disposed between capacitor electrodes, has increased.
Here, the high-dielectric means a dielectric having a dielectric constant higher than the dielectric constant κ=about 3.9 of silicon oxide (SiO2).
For example, in dynamic random access memories (DRAMs), conventionally, a material whose dielectric constant κ is up to about 10 (SiO2, SiN, or the like) is used as an inter-electrode dielectric. However, in recent years, a material whose dielectric constant κ is up to about 30 (TaO2, TiO2, Al2O3, ZrO2, HfO2, or the like) has come to be used, and moreover, a material whose dielectric constant κ is up to about 500 (BST, PZT, SBT, or the like) has come to be used.
Further, with respect to a ferro-electric memory (Ferro-electric RAM: hereinafter FeRAM), currently, a case has come to increase in which a ferro-electric material whose dielectric constant κ is from about 100 up to about 500, for example, PZT, SBT, or the like is used as an inter-electrode dielectric.
With respect to the ferro-electric memory, there is disclosed in, for example, Patent Document 1 (Jpn. Pat. Appln. KOKAI Publication No. 2000-36568) and Patent Document 2 (Jpn. Pat. Appln. KOKAI Publication No. 10-275897).
Here, electric lines of force generated between capacitor electrodes will be considered.
As shown in FIG. 1, at the central portion of the capacitor, the electric lines of force generated between the capacitor electrodes extend straightly from one electrode toward the other electrode. However, at the end portions of the capacitor, the electric lines of force trace a curve.
Here, as described above, currently, there is the trend that the dielectric constant κ of an inter-electrode dielectric of a capacitor becomes higher. On the other hand, as an insulating layer surrounding the capacitor, generally, silicon oxide (SiO2) is used.
In this case, for example, as shown in FIGS. 2 and 3, there are cases in which the electric lines of force greatly protrude from a space between the capacitor electrodes due to a difference between the dielectric constants of the inter-electrode dielectric (in the present example, PZT) and silicon oxide (SiO2) disposed at the periphery of the inter-electrode dielectric.
Such swelling of the electric lines of force at the end portions of the capacitor will be not especially a problem when the areas of the capacitor electrodes are sufficiently large with respect to the distance between the capacitor electrodes. This is because, in such a case, because most of the electric lines of force cross through the space between the capacitor electrodes, the amount of the loss in electrostatic energy due to the swelling of the electric lines of force can be ignored.
However, as the miniaturization of the elements progresses, and as the areas of the capacitor electrodes become smaller, the amount of the loss in electrostatic energy due to the electric lines of force being swelled out of the space between the capacitor electrodes come to be not able to be ignored. Namely, the capacitance of the capacitor is made to be small by the amount of the loss, and a sufficient capacitance cannot be ensured.
For example, in semiconductor memories using a capacitor as a data storage element (DRAMs, FeRAMs, and the like), because the data is stored as an amount of electric charge stored in the capacitor, such a reduction in capacitance means that a margin between “1” data and “0” data is made to be small, and a deterioration in the S/N ratio of the data will be brought about.
Note that a conventional FeRAM structure is shown in FIG. 4 as an example of a semiconductor memory using a capacitor as a data storage element. It is important for improvement in the S/N ratio that, in such a FeRAM, the loss in electrostatic energy due to the swelling of the electric lines of force at the end portions of the cell capacitor is made to be as little as possible, and in accordance therewith, the capacitance of the cell capacitor is improved.